Uniform momentum and temperature zones in unstably stratified turbulent channel flow

In wall-bounded turbulent flows, streamwise momentum organizes into uniform momentum zones (UMZs), separated by intense interfacial gradients. These structural features have received a significant amount of attention in neutrally-stratified flows, but the extent to which UMZs and corresponding uniform temperature zones (UTZs) are modified by unstable thermal stratification remains poorly understood. Using large eddy simulations of unstably stratified turbulent channel flow, we investigate the effects of buoyancy on UMZ and UTZ structure. Statistical properties of UMZs and UTZs (zone depths and interfacial velocity or temperature jumps) give rise to the scaling of the near-wall mean velocity and temperature gradients predicted by Monin-Obukhov similarity theory. Farther from wall, both UMZ and UTZ depth asymptote to a constant fraction of the outer length scale with negligible interfacial jumps, consistent with the well-mixed structure of the convective mixed layer. Conditional averaging reveals that ejections (sweeps) of momentum and warm updrafts (cool downdrafts) are prevalent below (above) UMZ and UTZ interfaces. These results demonstrate a connection between instantaneous structures, turbulence statistics, and scaling laws in these unstably stratified flows.